Hydraulic control apparatus for aircraft flight control apparatus



y 1962 P. F. HAYNER HYDRAULIC CONTROL APPARATUS FOR AIRCRAFT FLIGHT CONTROL APPARATUS Original Filed Oct. 25, 1954 3 Sheets-Sheet 1 b CRAFT AUIPOIMATIC CONTROL CRAFT CONTROL SURFACE INVENTOR.

PAUL F. HAYNER ATmRWE Y IE. Z

July 31, 1962 P. F. HAYNER 3,046,945

HYDRAULIC CONTROL APPARATUS FOR AIRCRAFT FLIGHT CONTROL APPARATUS Original Filed Oct. 25, 1954 3 Sheets-Sheet 2 INVENTOR.

PAUL F HAYNER ATTORNEY HYDRAULIC C ROL APPARATUS FOR AIRCRAFT July 31, 1962 P. F. HAYNER 3,046,945

FLI CONTROL APPARATUS Original Filed Oct. 25, 1954 I5 Sheets-Sheet 3 ATTURN 12 Claims. c1. 121-41 The present invention is concerned with hydraulic servo mechanisms for flight control apparatus, in particular, the flight control apparatus of the invention disclosed in copending application Serial No. 464,221, filed October 25, 1954, now Patent No. 3,011,740, the present application constituting a division of said copending application.

in flight control apparatus in which craft control surfaces normally are controlled by automatic flight control apparatus such as an automatic pilot, there is still a need for manual control of the craft control surfaces whenever the pilot. desires such or upon failure of the automatic control. Hydraulic servo mechanisms for such control apparatus must allow the pilot to take over control of the craft by manual means when he so desires or whenever such is necessary for proper craft control, that is, upon failure of the hydraulic servo pressure source, upon failure of the automatic flight control apparatus, or upon the necessity of overriding the automatic control apparatus.

The hydraulic servo mechanism in particular comprises an improved cut-ofi means for rendering the hydraulic boost actuator ineffective whenever there is a failure in the hydraulic fluid source so that the pilot will have complete manual control; and an improved override mechanism so that the pilot can manually override the automatically controlled servo mechanism.

It is therefore an object of the present invention to provide an improved hydraulic servo mechanism for use in craft control apparatus. 7

Another object of the present invention is to provide in a hydraulic servo mechanism an improved cutofl valve so that upon the application of fluid under pressure to the valve rapid operation will occur.

Another object of the present invention is to provide, in a hydraulic servo mechanism, an override means associated with a hydraulic actuator so that upon the existence of a fluid pressure on one side of the actuator higher than the supply pressure, fluid will flow from one side to the other of the actuator, thereby reducing the existing pressure.

Other objects of the invention will be apparent from a consideration of the accompanying specification, claims, and drawings, of which:

FIGURE 1 is a schematic view of a flight control apparatus containing the improved hydraulic servo mechanism,

FIGURE 2 is a sectional view of the pivotal connection between the main body and the base member of the flight control apparatus providing a continuous hydraulic connection between the base member and the main body,

FIGURES 3 and 3A are schematic sectional views of the hydraulic servo mechanism, and

FIGURE 4 is a sectional View of another embodiment of a hydraulic override of the type that is shown in FIG- URE 3A.

The flight control apparatus of the invention disclosed in the copending application mentioned above provides for the manual control of the aircraft control surface such as an elevator or rudder and superimposed on that control is the aircraft automatic pilot control which is normally provided by an electronic apparatus to maintain the craft on a course set by a selector. As it requires a con- 7 3,946,945 Patented July 31, 1952 siderable amount of effort to manually move the craft control apparatus, a hydraulic boost mechanism is pro-- vided to interconnect the manual control lever to the main body thereby reducing the amount of force normally required by the pilot to actuate the body and flight control apparatus.

In particular, referring to FIGURES 1 and 2, a main body 16 of a flight control apparatus is pivotally connected by a shaft 11 to a base member 12. A manual control lever 13 is connected by a link 14 and a crank 15 to the main body. valve 16 of an improved hydraulic boost servo mechanism 17 having an actuator 18, the output of which is connected by linkage 21 to the base member. The boost mechanism will be explained in detail later in the specification. An output arm 19 is also pivotally connected by shaft 11 to base member 12 and is connected to a craft control surcontrol lever can effectively operate the craft control surface even without the boost actuator mechanism in operation; however, with a hydraulic fluid under pressure supplied from a source (not shown) to conduits 26 and 28, 27 beingan exhaust conduit, connected to the body 10 through the shaft 11, the boost mechanism upon movement of crank 15 operates control valve 16 to cause actuator 17 to position the body 10 with respect to the base member 12. I

Another improved hydraulic servo mechanism 40 has a control valve 41 assembly that is connected to an actuator 42 whose output is connected by a link 43 to output arm 19. When valve 41 assembly is operated by a solenoid 44 in response to a craft automatic pilot control 45, output arm 1 (with the lock means inoperative) is able to move with respect to the body to further position the craft control surface in response to the automatic pilot control signal.

Referring to FIGURE 2, shaft 11 is shown rigidly connected to base member 12 and body 10 is pivotally connected to the shaft as well as crank 19. A sleeve or hearing 50 surrounding shaft 11 contains a plurality of sets of annular grooves, two of which are, as examples, 51 and 52 located on the inner and outer surface of the sleeve, respectively. A series of holes 53 through the sleeve interconnects each set of grooves. Conduits 26 and 27 (28 not shown) are each connected to a particular inner groove so that regardless of the position of the body with respect to shaft 11 continuous hydraulic communication is maintained from shaft 11 to body 10, the servo mechanism of the body being connected to conduits 54, 55 and 56, each of which connect to a respective outer groove.

In FIGURE 3 the boost servo mechanism 17 is shown with control valve 16 being operated by crank 15. The valve comprises a movable member 60 biased against the crank by a spring 61. A counterbalance 62 on the opposite side of crank 15 from the movable member provides for an equal pressure on the crank for centering.

Valve lfiisconnected to actuator 18 by passages 63 and 64, the actuator having a movable piston 65 connected by piston rod 66 and link 21 to base member 12. Fluid under high pressure is received by valve 16 from conduit 26 through a passage 70, and exhaust is provided by a passage 71 connected to conduit 27, Movement ofcrank 15 to position movable member 60 of valve 16 controls the supply of pressure to one side or the other of piston 65 to move body It with respect to base member 12 in response to the manual control signal received by crank 15. Thus it is seen that the pilot has manual control of the craft control surface by the control lever; however to reduce the force required by the pilot to operate such The crank 15 also controls a controlv ily flows from the chamber on either side of piston 65 to the other through passages 63 and 64. When member 74 is displaced to the left, passage 63 is opened, communication between 63 and 64 is prevented and passage 64 remains open because of the annular groove in the bore formember 74.

A snap action valve 80 controls valve 73. Valve 86 has a movable member 81 having pistons 82 and 83 with different eifective areas. A cavity 84 between the pistons is connectedto control valve 73 by'a passage 85 so that hydraulic fluid can be applied to the end of movable member '74 opposite spring 75. Normally, member 81 is biased to the left bya spring 90 so that piston 83 closes a connection to high pressure'passage 70 and piston 82 uncovers a' connection to exhaust passage 71. Upon the application of hydraulic fluid under pressure from passage 70 tothe left end of the smaller piston member 82 opposite spring 90, the force on movable member 81 resulting from the pressure of the hydraulic fluid on piston 82 begins movement of member 81 to the right against the force of spring 90 to initially close off exhaust passage 71. Upon further movement of member 81, piston 83 opens high pressure passage 70 to permit the fluid to exert its force on the larger piston 83, thereby accelerating the movement of member 81 to the right as well as permitting high pressure fluid to enter cavity 84 and passage 85. Upon the existence of high pressure fluid in passage 85, movable member 74 of control valve 73 will move to the left against spring 75 to open passage '63 and disconnect the interconnection between the opposite sides of the actuator '18 thereby providing normal control of the actuator 18 by control valve 16. The valve 80 insures an immediate and positive snap-action operation of valve 73 when fluid under pressure is available, thus immediately placing the boost mechanism in operation.

The hydraulic servo mechanism of FIGURE 3 used to control output arm 19 in response to the craft automatic pilotcontrol has a control valve assembly 41 of a type disclosed in the application of Paul F. Hayne-r et al., Serial No. 404,684, filed January 18, 1954, now abandoned, comprising a first valve 101 having a movable member 101 positioned by the resultant effect of a pair of solenoids 102 and 103 interconnected by a pivotal link member 104. When electrical power is supplied to either one of; the solenoids by a connection (not shown) from the craft automatic pilot control, member 101 moves either to the left or right by an amount depending on the energization of the solenoids. Member 161 is. surroundedby a movable sleeve 110. Movable member 101 isfbiased to the right ,by a spring 111. A stop 117 limits the leftward movement of the movable member 101. ,Another stop 118 associated with link 104 limits the movement of member 101 to the right. Connected to the valve 100 is a control valve 112 having. a movable member 113'; A cavity 114 between pistons 115 and 116 of movable member 101 is. connected by a passage 120 to activity 121 at one end of movable member 113. Normally, pistons 115'and-1v16 shut off the flow of fluid from 'cavity 114-to the exhaust passage 71 and from high pressure passage 70 to cavity 114, respectively. Movement 114 as well as in cavity 121 and movement of the mem f extremity of bore 166 near the outer extremity of mova- V A a of member 101 to the left exhausts the fluid in cavity AL ity 121. Sleeve and the end of movable member 113 remote from cavity 121 are connected by a pivoted lever 122 so that the position of member 113 establishes a corresponding position of sleeve 110 in response to the pressure in chamber 121 for follow-up purposes While force through lever 122 is rightwardly applied to sleeve 110, a counter acting force is applied to the sleeve by high pressure fluid-of passage 70 acting on the righthand end area of the sleeve. 7

An actuator 42, referring to FIGURE 3A, comprising a piston 131 mechanically connected to crank 19 by piston rod 183 and link 43 is hydraulically connected to control valve 112 by a pair of passages 132 and 133 connected to the opposite sides of piston 131. Movable member-1'13 of control valve 112 comprises three pistons 135, 136 and 137, piston 136 normally closing off the passage of high pressure servo fluid from conduit 28 through a passage 140. Passages 132 and 133 are connectedto the cavities between pistons and 136, and '136 and 137,

respectively, so that as movable member 113 is moved in either direction high pressure fluid is supplied from" passage 14% to the respective cavity and thereby to either one side or the other of piston 131. Pistons 135 and 137 normally block exhaust passages 141and 142 that are connected to exhaust passagejl and thus conduit 27.

interposed between control valve 112 and actuator 42 is a cutoff valve 145 the operation of which is similar to and 133 exists so that piston 131 can be moved. Upon the existence of a supply of hydraulic fluid under pressure in conduit 28, fluid is applied to passage 149 and to piston 154 at one end of movable member 150 having a relatively small effective area so that movable member 151) is moved to the right against the spring at a relatively slow rate. Subsequently, another piston 155 having a larger effective area uncovers a connection to passage 141) to apply fluid under pressure to piston 155 thereby. moving member 150 to the right at a greater rate. With movable member 159 to the right, the interconnection between conduits 132 and 133 is broken by the repositioning of 'piston 152. Thus, actuator 42 is connected to be controlled by control valve 112. Cutofi valve 145 insures a positive connection between the control valve and the actuator, this connection is quickly made whenrthe supply of fluid under sufficiently high pressure is avail-able.

Connected in parallel with actuator 42 is an override valve 161. Valve 161 comprises movable members 162 and 163 arranged adjacent to each other for axial movement within a bore 166 in body 10. Movable member 163 comprises two pistons 164 and 1 65 and member 162 has a single piston 179. One side of actuator 4-2'is con-' and 165. by a passage 172. A passage r173 normally blocked by piston 170 is connected to cavity 168. High presssure fluid is admitted to cavity 174 at the left hand ble member 162 from passage 140 and this pressure normallymaintains movable members 162 and 163 to the right against a stop. When the pressure on one side of actuator '42 exceeds the pressure of the source, the actuator pressure is applied to override valve 161 by either passage 171 or 172. When the high pressure exists in passage 171, movable member 162 is moved tothe left and piston 170 ropens'passage 173 to allow 'fiuid to flow from passage 171 to passage 172 until thepressure in passage 171 is below the supply pressure. Should the pressure" in passage 172'exceed the supply pressure of passage; 140, movablemember 163 will move to the left-pushing member '162 to also uncover passage 173 and allow fluid to flow from 172 to 171, this operation of 163 being possible due to a passage 163- in piston 165 as a pressure is applied to the right end of piston 165. It is therefore seen that a manual control of body can override any superimposed automatic signal to position the craft control surface independent of automatic signals.

A centering mechanism 180 is provided to center the piston 131 when high pressure fluid is not available. The centering mechanism comprises a pair of dished members 181 and 182 mounted on a shaft 133 connected to piston 131 and held apart against raised portions on the shaft by spring 184. Normally, a chamber on the outer side of each dish member is connected by passage 185 and the cavity between pistons 152 and 154 of valve 145 to exhaust passage 71; however, upon the application of hydraulic fluid under pressure to valve 145, piston 154 blocks the exhaust passage and connects passage 185 to the high pressure supply. When high pressure fluid is applied to the outer sides of each of the dished members, they are moved toward one another against spring 184 to permit movement of shaft 133. Upon the failure of the source of hydraulic fluid, members 181 and 132 will move outwardly returning piston 131 to the center of actuator 42 which in turn repositions output arm 19 in a center position with respect to body 1%.

Normally, without a supply of hydraulic fluid under pressure. output arm 19 is locked in place with respect to body 10 by a pair of members 190 and 191 wedged between output arm 19 and the body by a pair of springs 192 and 193. Upon the application of high pressure servo fluid to the centering device through passage 185, fluid is also supplied to the locking device through a passage 194 and is directed to cavities on the underside of members 199 and 191 to move them upward against the springs 192 and 193 rendering the locking device in effective.

Referring to FIGURE 4, another embodiment of the override valve is shown. An actuator 195 having piston 131 is normally connected to the control valve (not shown) by passages 132 and 133. The override valve is contained in a hollow cavity 2% inside the rod 183. In the cavity, movable members 216 and 211 are adjacently located in axial relation and held to the right by the pressure of the source of hydraulic fluid applied at the connection 212. When the pressure in cavity 213, on one side of piston 131, exceeds the supply pressure, this high pressure from cavity 213 is applied through a passage 215 to movable member 2119 to move it to the left. Movement of member 2113 to the left uncovers a passage 229 to allow fluid to flow from cavity 213 through a passage 220 and 221 to the other side of piston 131, that being cavity 214. When the opposite is experienced and the pressure in 214 exceeds the pressure of the source this high pressure is applied to movable member 211 through a passage 222 to move both movable members against the supply pressure to also open passage 220 and allow hydraulic fluid to flow from the cavity 214 to the cavity 213. With such an actuator connected to the automatic flight control it is also possible for the pilot by manual control, either with boost servo action or without, to override the automatic operation whenever such is deemed necessary.

Operation As shown in FIGURES 3 and 3A the control apparatus is assumed for the moment to be only manually operative as there is no hydraulic pressure available from the source (not shown) connected to conduits 26 and 28. In this condition, the boost mechanism actuator 18 is ineflective and piston 65 is free to move back and forth placing relatively little resistance between the body 10 and base member 12. Hydraulic servo actuator 42 is centered by the centering mechanism 180 and output arm 19 is locked in the center position so that the pilot can manually control thecraft control surface by the 6 control lever through the connection comprising link 14, body 10, and link 25. Movement of crank 15 with respect to body 11 is limited by stops 220 so that with the boost system inoperative a substantial connection exists between link 14 and body 10. While this sort of conitrol would be successful, it would normally be only used during emergency as there would be a considerable effort needed to manually control such a craft control surface not only due to the wind resistance upon changing the position of a rudder or elevator but also due to the frictional resistance in the connection between the control lever and such elevators or rudders.

With fluid under pressure available through conduits 26 and 28, conduit 27 being connected to the exhaust of the fluid source, immediately movable member 81 of valve 3% would move to the right to insure a positive connection between passage 85 and passage 70, admitting high pressure fluid to the right extremity of movable member of control valve '73. The instantaneous operation of control valve 73 would positively place the boost servo mechanism in operation so that upon movement of manual control lever 13 and-a subsequent operation of control valve 16, piston 65 of actuator 18 would provide the driving power necessary not only to move body 10 with respect to the base member but also to maintain the craft control surface in the position selected by the manual control lever 13. When link 14 moves to the right, hydraulic fluid under pressure is admitted to the right side of piston 65. Body 10 then moves to the right clockwise about pivot 11 (see FIG- URE 1). The opposite occurs when link 14 is pulled to the left. With the boost servo mechanism in operation a relatively small amount of force is needed by the pilot to control the craft control surface.

The availability of high pressure fluid from conduit 28 places hydraulic servo mechanism 40 used to automatically control the position of output arm 19 into operation. Movement of member of-cutoff valve 145 to the right upon the application of high pressure fluid at its left extremity not only renders the operation of actuator 42 effective in response to the operation of control valve 41 but also deenergizes the lock means holding output arm 19 rigidly in position and renders centering mechanism inoperative. The hydraulic servo mechanism is thus able tocontrol the position of output arm 19 with respect to the body as determined by the electrical signal received from the craft automatic pilot control which operates the control valve solenoids 102 and 1113.

Upon the instantaneous operation of the cut-off valve 145, the control valve 41 is connected in a controlling relation to the actuator '42. Movement of the movable member 101 in response to the output signal of the automatic pilot control 45 results in movement of the control valve 112. If member 101 is moved to the right, high pressure fluid enters the space 121 through the passage 70 to cause member 113 to move to the left thereby admitting high pressure fluid to one side of the piston 131 through passages 140 and 133, the other side of the piston being exhausted through passages 132 and 141. This causes movement of output arm 19 with respect to body 16 as body 10 is held under the power of servo 17, its actuator 18 being coupled to base 12. At the same time, sleeve 119 is moved to the right against the pressure of the fluid from passage 70 by the force exerted through link 122 from the movable member 113, this acting as a feedback and stabilization means for the valve. Under normal operation, the control valve posi tions the actuator 42 to. move the output arm 19 in response to the signal received by the solenoids from the craft automatic pilot control 45.

Assuming that a maximum signal was received from automatic pilot control 45 to move output arm 19 counterclockwise to the right with respect to body 10 as far as actuator 42 would allow. The pilot, desiring to move valve as shown in FIGURE 4 is similar.

control lever 13 to the left, would not be aware of the fact that a maximum amount of operation of the craft control surface in that direction has already been accomplished by the automatic control. Such often results in a straining of the linkages and. even damage to the manual control mechanism for the pilot may believe he still has more control over the craft control surface, as he has not changed the position of control lever to its leftward extremity. To prevent such overstressing, over ride valve 161 is provided.

Whenever a manual movement of body 10 is desired andthe craft control surface is at either of its extremities as a result of a signal from the craft automatic pilot control, valve 161 allows repositioning of piston 131 to relocate the body 10 allowing output arm 19 to remain stationary. .With output arm 19 to the right as previously assumed, piston 131 is to the right and any move' ment of body 1 counterclockwise bythe manual control lever 13 wouldtend to move piston 131 to the other end of actuator 42. As the fluid in the cavity on the left .side of piston 131 is compressed to a pressure greater than thatof the source available in passage 14%, a pressure differential exists across piston 170 of movable member 162causing the member to move to the left thereby opening passage 173 so that fluid can flow between passages 171 and 172 allowing piston 131 to move to the left. Due to the override valve 161, the manual control lever 13 could be moved to the left against a stop (not shown) without disturbing the relation between the automatic pilot control and the craft control surface. Similarly operation in the opposite direction would place a pressure'difierential across pistons 170 and 164 to move both member 162 and 163 to the left :again providing flow of fluid from opposite sides of actuator 42 through passage 173.

The operation of the other embodiment of the override The override valve is contained in an inner cavity in the shaft and piston assembly of the actuator, it having its particular advantages depending upon the actuator size requirements and other design features of the control.

While I have shown a specific form of hydraulic servo mechanism for control apparatus, it is to be understood that this is for illustrative purposes only and that the'invention is to be limitedsolely by the scope of the appended claims.

I claim as my invention:

1. 'Inflight control apparatus for aircraft, a main mem-' ber; a base member; pivotal connection means connecting said main member to said base member, said main member comprising input means for receiving a flight condition signal and output means for controlling a craft control means; automatic flight control means, said flight control means having control valve means being controlled in response to a second flight control signal; and actuator means being mechanically coupledto said output means, said ;control valve including a first valve unit having a plug and a sleeve both controlling flow through a fluid pressure'line and through an exhaustline, a second valve unit having a plug alternatively connect-able with said pressure or exhaust lines by said first valve unit, and means connecting the sleeve of said first valve and the plug of said second valve, sothat upon displacement of said plug of said first valve unit'to apply pressure to said plug of the secondvalve unit, movement of the plug of said second valve through the connecting means will apply a force to the sleeve of said first valve to restore it to its normal position with respect to the plug of the first valve unit.

2. In flight control apparatus for aircraft, a main member, a base member, and pivotal connection means connecting said main member to said base member, said main .membercomprising input means for receiving a first flight condition signal, a hydraulic servo' system receiving a second flight control signal, and output means for comprising a source of fluid under pressure,"a first valve 7 having a movable member and an associated sleeve for connecting a passage selectively to said source, a second valve having a second movable member biased in one direction by the pressure developed in said passage, said second valve connected in controlling relation to actuator means for said output means, and feedback means for connecting said second valve to the sleeve of said first valve so that an equilibrium can be established by the restoration of the sleeve and plug of said first valve in the proper relation.

3. In flight control apparatus for aircraft, a main member having input means and output means, pivotal connection means connecting said member to a base member providing continuous hydraulic communication between said base and said member, control means integral with said main member comprising an actuator having a piston connected to said output means, a source of fluid under pressure, control valve means connecting said source in a controlling relation to said actuator, automatic flight control means for controlling said control valve means to eifectively control craft control means connected to said output means, override means connected in parallel with said actuator, said override means comprising a pair of movable members .adjacently spaced in a cylinder, conduit means connecting said sourceto said override means so that fluid pressure is applied to one outer extremity of one of said movable members, second conduit means connecting one side of said actuator to the cavity between said members, third conduit means connecting the other side of said actuator to the outer extremity of the second of said movable members, and valve means including said one member for connecting said cavity to the other side of said actuator whenever said one member is moved to the left against said fluid under pressure.

4. In flight control apparatus for aircraft, a main unit having input means at one extremity connected to a manual control lever and output means at the other extremity connected to craft control means, said main unit responsive means, actuator means, a source of hydraulic 1 fluid under pressure, and means includingsaid'control valve means for connecting said source to said actuator means, said actuator having a piston connected to said output means so that the flight condition responsive means can superimpose its effect in addition to that of said C0111 trol lever on said output means, and override means comprising a two-way valve means connected in parallel across said piston, said valve providing for flow of fluid from one side of said piston to the other when the pressure developed on said one side exceeds the pressureof said source.

5 In a hydraulictransmission system for flight control apparatus, control valve means, actuator means, a source of fluid under pressure, connection means including said control valve means connecting said source to saidactuator means, condition responsive means connected in con-' trolling relation to said control valve means, condition changing means operable in either direction from a normal position, connection means connecting said actuator; in

controlling relation to saidcondition changing means, and override valve means including a member displaceeither of saidsides exceeds the pressure of, said'sourcen 6 In hydraulic transmission apparatus, control valve. means, actuator means, a source of fluid under pressure,

connection means including said control valve means for connecting said source to said actuator means, override valve means connected in parallel with said actuator, said override valve means comprising first and second members 9 adjacently aligned in a bore, conduit means connecting said source to one extremity of said bore adjacent said first member, second conduit means connecting one side of said actuator means to the other extremity of said bore, third conduit means connecting the other side of said actuator to said bore intermediate said members, valve means comprising said first member for connecting second and third conduits when said first member is moved in said one direction upon the pressure on either side of said actuator means exceeding the pressure of said source.

7. In a piston type hydraulic servo system comprising control valve means, a source of fluid under pressure, piston actuator means, and conduit means including said source for connecting said control valve means in controlling relation to said actuator, and valve means biased in one direction from said source and connected in parallel with said actuator means and subject to a force opposing said bias for equalizing the pressures in said actuator means whenever said pressures exceed the pressure of said source.

8. In control apparatus, a main member having input means and output means, said input means including boost servo means having control valve means and actuator means for providing an added source of power when moving said main member; cutcif valve means connecting said control valve means and said actuator means to selectively render said boost means ineffective, automatic control means associated with said output means for controlling said output independently of said input means; said control means comprising valve means having feedback means for maintaining said valve means operative; override means associated with said control means so that said input means can overpower said control means to position said output means irrespective of said control means; and pivotal connection means connecting said main member to a base member to provide continuous hydraulic communication between said members.

9. In flight control apparatus, comprising a main body, a base member, pivotal connection means connecting said body to said base member, control means, condition changing means, connecting means including said body for connecting said control means to said condition changing means, said pivotal connection means comprising a shaft having a plurality of passages connected to a source, a sleeve surrounding said shaft having a plurality of annular grooves each connected in communication with said passages, aud a plurality of passages in said body each being connected to its respective groove thereby maintaining a plurality of separate passages for communication between said source and said body.

10. In flight control apparatus for aircraft, a main body pivotally supported on a base member, said body having input means and output means; servo boost means connected between said body and said base member, said boost means operative in response to said input means; said output means including an output arm adapted to be connected in a controlling relation to a craft surface control; servo means having an output shaft connected between said body and said output arm, said servo means adapted to be operated in response to an automatic flight controller to control said craft surface through said output shaft independent of said input means; and centering means comprising a pair of dished members mounted on said shaft, resilient means biasing said members apart against respective shoulders on said shaft thereby holding said shaft in a predetermined position for centering said output arm with respect to said body, and pressure operated means for forcing said members together against the opposing forceof said resilient means to permit movement of said shaft.

11. In flight control apparatus, comprising a main body, a base member, a pivotal connection means connecting said body to said base member, control means, condition changing means, a connecting means including said body and pivotal connection means for connecting said control means to said condition changing means, said pivotal connection means comprising a shaft having an axial and radial passage connected to a source, a sleeve surrounding said shaft, said sleeve having an internalcircumferential groove and an external circumferential groove with a plurality of circularly spaced passages connecting said grooves, the internal groove axially being aligned with -the radial passage in said shaft, a passage in said body communicating with said external groove thereby maintaining communication between said source and said body during relative "angular movements of said shaft and main body.

12. In a piston type hydraulic servo system comprising a control valve means, conduit means connecting said control valve means to a source of pressure, piston actuator means, means connecting said control valve means in conirolling relation to said piston actuator, said actuator being displaceable in either direction from a normal position upon difierential pressure upon opposite sides of the piston, and pressure equalizing displaceable valve means biased in one direction from said pressure source and connected in parallel with the opposite sides of the piston actuator means and subject to a force therefrom opposing said bias for equalizing the pressures on opposing sides of said piston actuator means whenever a pressure on either side thereof exceeds the pressure of said source.

References Cited in the file of this patent UNITED STATES PATENTS 2,030,986 Havill Feb. 18, 1936 2,316,235 Gast Apr. 13, 1943 2,597,419 Westbury May 20, 1952 2,771,062 Hayner Nov. 20, 1956 

